1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a plurality of fuel cell units that are formed by sandwiching an electrode assembly between separators.
2. Description of the Related Art
There is one type of fuel cell unit that is formed in a plate shape by sandwiching between a pair of separators an electrode assembly that is formed by placing an anode electrode and a cathode electrode respectively on either side of a solid polymer electrolyte membrane. A fuel cell is formed by stacking a plurality of fuel cell units that are structured in this way in the thickness direction of the fuel cell units.
In each fuel cell unit, there are provided a communication passage for fuel gas (for example, hydrogen) on one surface of the anode side separator that is located facing the anode electrode, and a communication passage for oxidizing gas (for example, air that contains oxygen) on one surface of the cathode side separator that is located facing the cathode electrode. In addition, a communication passage for a cooling medium (for example, pure water) is provided between adjacent separators of adjacent fuel cell units.
When fuel gas is supplied to the electrode reaction surface of the anode electrode, hydrogen is ionized at the electrode reaction surface and moves to the cathode electrode via the solid polymer electrolyte membrane. Generated electrons are extracted to an external circuit and used as direct current electrical energy. Because oxidizing gas is supplied to the cathode electrode, hydrogen ions, electrons, and oxygen react to generate water. Because heat is generated when water is created at the electrode reaction surface, the electrode reaction surface is cooled by a cooling medium made to communicate between the separators.
The fuel gas, oxidizing gas (generically known as reaction gas), and the cooling medium each need to travel through a separate communication passage. Therefore, sealing technology that keeps each communication passage sealed in a fluid-tight or airtight condition is essential.
Examples of portions that need to be sealed are: the peripheries of penetrating supply holes formed in order to supply and distribute reaction gas and cooling medium to each fuel cell unit of the fuel cell; the peripheries of discharge holes that collect and discharge the reaction gas and cooling medium that are discharged from each fuel cell unit; the outer peripheries of the electrode assemblies; and the outer peripheries and the like of the areas between the separators of adjacent fuel cell units. A material that is soft yet also has the appropriate resiliency such as organic rubber is employed for the sealing member.
In recent years, however, size and weight reduction as well as a lowering in the cost of fuel cells have become the main aims expected to lead to the more widespread application of fuel cells through their being mounted in actual vehicles.
Methods that have been considered for reducing the size of a fuel cell include making each fuel cell unit forming the fuel cell thinner, more specifically, reducing the size of the dimension between separators while maintaining a maximum size for the reaction gas communication passage formed inside each fuel cell unit; and also making the separators thinner.
However, a limit is imposed on how thin the separators can be made by the strength requirements for each separator and by the rigidity requirements for the fuel cell. Moreover, reducing the height of the sealing member is effective in reducing the size of the dimension between separators, however, the height of the sealing member needs to be sufficient for the sealing member to be able to secure a sufficient crushed margin in order to ensure the required sealing ability is obtained. Therefore, there is also a limit to how much the height of the sealing member can be reduced.
Furthermore, in a fuel cell unit, although the space occupied by the sealing members is indispensable for the reaction gas and cooling medium to be sealed in, because this space contributes substantially nothing to power generation it needs to be made as small as possible.
FIG. 24 is a plan view showing a conventional fuel cell. In FIG. 24 the symbol 70 indicates a communication hole such as a fuel gas supply hole and discharge hole, an oxidizing gas supply hole and discharge hole, and a cooling medium supply hole and discharge hole that each penetrate the fuel cell in the direction in which separators 71 are stacked. The symbol 72 indicates an area formed by a plurality of fuel gas communication passages, oxidizing gas communication passages, and cooling medium communication passages running along the separators 71.
FIG. 25 is a vertical cross-sectional view of a conventional fuel cell 73 taken along the line X—X in FIG. 24. As can be seen in plan view, in order to make the space occupied by the sealing member (which doesn't contribute to power generation) as small as possible, conventionally, by locating gas sealing members 76 and 77, which respectively seal a fuel gas communication passage 74 and an oxidizing gas communication passage 75, together with a cooling surface sealing member 78, which seals a cooling medium communication passage, in a row in the stacking direction of the fuel cell unit 79, the outer dimensions of the fuel cell 73 are kept to a minimum.
However, the drawback with the fuel cell 73 that is structured in this manner is that if the gas sealing members 76 and 77 that seal the communication passages 74 and 75 as well as the cooling surface sealing member 78 are all placed in a row in the stacking direction of the fuel cell unit 79, then the thickness of the fuel cell 73 cannot be made less than a value obtained by adding the height of the cooling surface sealing member 78 between fuel cell units 79 to the thickness of each fuel cell unit 79, and multiplying this result by the number of fuel cell units stacked in the fuel cell.
More specifically, as shown in FIG. 25, the fuel gas supply hole 70 and the fuel gas communication passage 74 that are isolated in a sealed state by gas sealing members 76 and 77 are connected by a communication passage 80. The communication passage 80 is provided in the separator 81 so as to detour around, in the thickness direction of the separator 81, the vicinity of the fuel gas supply hole 70 of the gas sealing member 77 that seals the entire periphery of the fuel gas communication passage 74. Moreover, the separator 82 also has a similar communication passage (not shown) in the oxidizing gas supply hole (not shown).
Accordingly, each of the separators 81 and 82 are formed comparatively thickly in order to form the communication passage 80, however, as is seen in the cross section in FIG. 25, at the position of the seal line where each of the sealing members 76 to 78 are placed, the separators 81 and 82 are formed with the minimum thickness needed to ensure the required strength, and it is not possible to make them any thinner.
Moreover, because each of the sealing members 76 to 78 are formed with the minimum height needed to ensure sealing ability, it is not possible to reduce the height of the sealing members 76 to 78 any further.
As a result, although the thickness of the fuel cell 73 is found by multiplying the number of stacks of fuel cell units by the sum of the minimum thickness of the two separators 81 and 82, the thickness needed to form the communication passage 80, the height of the two gas sealing members 76 and 77, the thickness of the solid polymer electrolyte membrane 83, and the height of the cooling surface sealing member 78, because these are all indispensable it is extremely difficult to achieve any further reduction in the thickness of the fuel cell 73.